Sigh . . .
Its been one of those days.  I screwed up on my last post of the 16 bit fast
counter.  Change the 'goto $+2' jumps in the normalise routine to 'goto $+3'.
Sorry.  I think that I'll go get some sleep.

;measure puse width on a 12c508 with 5 uSec resolution.
;Times out after about 327 mSec.

        clrf    c_low
        clrf    c_high

s1:
        BTFSC   IOPORT,IOBIT    ;5 cycles
         goto   went_high

        INCFSZ  c_low,F
         goto   s1

        clrwdt                  ;

        BTFSC   IOPORT,IOBIT    ;5 cycles
         goto   went_high

        INCFSZ  c_high,F
         goto   s1

;note: each time HI byte increments, LO byte to misses a count
;final count == LoByte + HiByte

time_out:
;do time out stuff here

went_high                       ;fix up the mod 257 counter

        movfw       c_high      ;
        addwf       c_low,F
        skpnc                   ;no carry - leave Hi byte alone
          incfsz    c_high,F    ;carry - does Hi byte contain ff?
            goto    $+3         ;either no carry or did not wrap
        movwf       c_low       ;wrapped past ffff - hold at ff
        movwf       c_high      ;ditto

The logic is simple - if the high byte contained ff and a carry was
generated when the 2 bytes were added together (which would wrap the high
byte to 00), the value in W (which could only be ff) is written to both bytes.

My thanks to all who contributed ideas and suggestions.  Scott - Thank you
again for taking time out of your very busy day.

dwayne